Fire alarm systems utilizing a UV detector tube as the fire sensor are well known. In many of the prior systems the UV detector tube is operated in the normal on-off manner and, consequently, such systems cannot distinguish between fires of different sizes or fires located at different distances from the tube. Also, many prior systems cannot distinguish between UV radiations emitted by a fire and those emitted by sources other than a fire. It is important to eliminate false alarms particularly in systems which automatically actuate fire extinguishers. A particularly troublesome cause of false alarms in existing systems in an electrical welding arc as such arc emits massive amounts of non-flickering UV radiations. Also, lightning flashes may cause the system to give a false alarm even though the resulting UV radiations are of relatively short duration.
An ultraviolet fire detection system made in accordance with this invention distinguishes between fire and other sources producing UV radiations. Also, the system is programmable to detect a fire of predetermined size located a predetermined distance from the UV detector tube, and, the system does not respond to high level, constant UV radiations produced by a welding arc.